Conjugated polymers have semiconductor characteristics and, therefore, are noted as semiconductor materials in place of known silicon and compound semiconductors. If such a polymer can be used as a semiconductor material, significant cost reduction can be expected because of inexpensiveness of a material and simplicity of a device manufacturing process compared with those of inorganic semiconductors, e.g., crystalline silicon and amorphous silicon, now in use. Furthermore, high-temperature processes become unnecessary and devices can be prepared by the use of coating technologies and printing technologies. Therefore, devices can also be provided on organic films, and driving devices of flexible displays and semiconductor devices of wearable computers are realized.
Inorganic semiconductors, e.g., crystalline silicon and amorphous silicon, are used for photovoltaic devices now. However, the costs of solar cells prepared by the use of these inorganic semiconductors are higher than those of power-generation types, e.g., thermal power generation and nuclear power generation, and solar cells have not yet come into adequately widespread use. A primary factor responsible for the high cost is the process in which semiconductor thin films are manufactured in a vacuum at a high temperature. Consequently, development of an inexpensive manufacturing method allows significant cost reduction and, thereby, fast expansion of the solar cell market can be expected.
With respect to semiconductor materials, in general, carriers (electrons or holes) included in the materials are required to have high mobilities. However, conjugated polymers have a problem in that the mobilities are lower than those of inorganic crystalline semiconductors or amorphous silicon. Consequently, semiconductor devices, e.g., Field Effect Transistors (hereafter referred to as FETs) and photovoltaic devices, including conjugated polymers have a problem in that response times and output currents are inadequate. Therefore, application areas of organic semiconductors are significantly limited now and, in general, inorganic compounds, e.g., crystalline silicon, gallium arsenide, and amorphous silicon, are used under the present circumstances.
With respect to a FET device, it is preferable that larger current flows between a source electrode and a drain electrode. The current becomes larger as the mobility becomes higher. For example, the current Is in the region at which a current flowing between a source electrode and a drain electrode becomes saturated relative to a gate voltage (hereafter referred to as a saturation current) is represented by the following equation.Is=(μCW/2D)(Vg−Vth)2  (1)
Here, μ represents a mobility, C represents a capacitance of an insulating material on the gate electrode, D and W represent a distance between the source electrode and the drain electrode and an electrode width, respectively. Vg represents a gate voltage, and Vth represents a gate voltage at which a saturation current begins to flow. As is clear from the equation (1), the saturation current of the FET increases as the mobility μ of the semiconductor material becomes higher.
The maximum operating frequency fm of the FET is roughly represented by the following equation, and driving can be carried out at a higher frequency as the mobility becomes higher.fm=μV/2πD2  (2)
Here, μ represents a mobility, V represents a voltage between the source electrode and the drain electrode, and D represents a distance between the source electrode and the drain electrode.
On the other hand, with respect to the organic photovoltaic device including a conjugated polymer, the largest problem is that the photoelectric conversion efficiency is lower than those of known inorganic semiconductors and, therefore, commercialization is not yet realized. The primary reasons for this are the facts that traps which capture carriers tend to be formed in organic semiconductors and, thereby, formed carriers tend to be captured by traps and the mobilities of the carriers are decreased, and that a bound state referred to as an exciton is formed, in which an electron and a hole formed by incident light are not readily separated from each other.
It is an object of the present invention to provide a method for increasing the mobilities of carriers in an organic semiconductor material constituting a semiconductor device, e.g., a transistors and a photovoltaic device, an organic semiconductor material prepared by the method, and an organic semiconductor device prepared by the use of the material.